JP7628072B2 - Water-based paint composition and method for producing coated object - Google Patents

Description

The present invention relates to an aqueous coating composition and a method for producing a coated article.

Conventionally, blocked isocyanates have been used as hardeners for one-component water-based paints from the viewpoint of the physical properties of the resulting coating film. However, paints containing blocked isocyanates may not have sufficient storage stability.

Patent Documents 1 and 2 propose using a carbodiimide compound as a curing agent for one-component water-based paints or water-based resin compositions.

JP 2021-143320 A JP 2018-104605 A

However, the aqueous paint of Patent Document 1 has poor storage stability. The aqueous resin composition of Patent Document 2 may have poor water resistance. Furthermore, the aqueous paint composition of Patent Document 2 is intended for use on disposable articles, and is not intended for long-term outdoor use. The present invention has been made in view of the above, and aims to provide a one-liquid aqueous paint composition that has excellent storage stability and can be cured at low temperatures, and a method for producing a coated article that has excellent water resistance.

（式中、Ｘは、それぞれ独立して、少なくとも１個のカルボジイミド基を含有する２官能性有機基であり、Ｙは、それぞれ独立して、ポリアルキレングリコールモノアルキルエーテルから水酸基を除いた構造を有し、Ｚは、数平均分子量２００以上５，０００以下の２官能ポリオールから水酸基を除いた構造を有する。）、
下記一般式（ＩＩ）：

（式中、Ｘは、それぞれ独立して、少なくとも１個のカルボジイミド基を含有する２官能性有機基であり、Ｙは、それぞれ独立して、ポリアルキレングリコールモノアルキルエーテルから水酸基を除いた構造を有し、Ｒ^０は、水素、メチル基、またはエチル基であり、Ｒ^１は、それぞれ独立して、炭素数４以下のアルキレン基であり、ｎは０または１であり、ｍは０～６０である。）、および、
下記一般式（III）：

（式中、Ｘは、それぞれ独立して、少なくとも１個のカルボジイミド基を含有する２官能性有機基であり、Ｙは、それぞれ独立して、ポリアルキレングリコールモノアルキルエーテルから水酸基を除いた構造を有する。）
で表される化合物よりなる群から選択される少なくとも１種を含む、上記［１］～［９］のいずれかに記載の水性塗料組成物。
［１１］
上記［１］～［１０］のいずれかに記載の水性塗料組成物を被塗物に塗装して、未硬化の塗膜を形成する工程と、
前記未硬化の塗膜を７５℃以上１００℃以下の温度で加熱して、硬化された塗膜を得る工程と、を備える、塗装物の製造方法。 In order to solve the above problems, the present invention provides the following aspects.
[1]
A one-component aqueous coating composition,
(A) an aqueous resin having a hydroxyl group and a carboxyl group;
(B) a hydrophilically modified carbodiimide compound;
(C) a basic compound,
The glass transition point of the (A) aqueous resin is 0° C. or higher and 20° C. or lower,
The hydrophilically modified carbodiimide compound (B) has at least one carbodiimide group and a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether in the molecule,
The pH of the aqueous coating composition is 8.5 or more and less than 10,
The aqueous coating composition does not contain a polyisocyanate curing agent or contains less than 1 part by mass of a polyisocyanate curing agent per 100 parts by mass of the solid content of the aqueous coating composition.
[2]
The aqueous coating composition according to the above-mentioned [1], wherein the content of the (C) basic compound is 1 part by mass or more and 10 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition.
[3]
The aqueous coating composition according to the above [1] or [2], wherein the boiling point of the basic compound (C) is 180° C. or lower.
[4]
The aqueous coating composition according to any one of the above [1] to [3], wherein the (C) basic compound comprises at least one selected from the group consisting of ammonia and amine compounds.
[5]
The aqueous resin (A) has a hydroxyl value of 20 mgKOH/g or more and 100 mgKOH/g or less, and an acid value of 5 mgKOH/g or more and 80 mgKOH/g or less, calculated on a resin solid content basis. The aqueous coating composition according to any one of [1] to [4] above.
[6]
The aqueous coating composition according to any one of the above [1] to [5], wherein the ratio of the equivalent weight Ec of the carbodiimide group of the hydrophilically modified carbodiimide compound (B) to the equivalent weight Ea of the acid group of the aqueous resin (A): Ec/Ea is 0.1 or more and 1.5 or less.
[7]
The content of the (A) aqueous resin is 30 parts by mass or more and 75 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition. The aqueous coating composition according to any one of [1] to [6].
[8]
The content of the hydrophilically modified carbodiimide compound (B) is 5 parts by mass or more and 15 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition. The aqueous coating composition according to any one of [1] to [7].
[9]
The hydrophilically modified carbodiimide compound (B) is represented by the following general formula (α):
-OCONH-X-Q-Y (α)
(In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, each Y is independently a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether, and Q is -NHCOO- or NHCONH-.)
The aqueous coating composition according to any one of the above [1] to [8], having one or more structural units represented by the following formula:
[10]
The hydrophilically modified carbodiimide compound (B) is
The following general formula (I):

(In the formula, X's are each independently a bifunctional organic group containing at least one carbodiimide group, Y's are each independently a polyalkylene glycol monoalkyl ether having a structure in which a hydroxyl group has been removed, and Z's are each independently a bifunctional polyol having a number average molecular weight of 200 or more and 5,000 or less having a hydroxyl group removed.)
The following general formula (II):

(wherein, X's are each independently a bifunctional organic group containing at least one carbodiimide group, Y's are each independently a polyalkylene glycol monoalkyl ether having a hydroxyl group removed therefrom, ^R0 is hydrogen, a methyl group, or an ethyl group, ^R1 's are each independently an alkylene group having 4 or less carbon atoms, n is 0 or 1, and m is 0 to 60), and
The following general formula (III):

(In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, and each Y is independently a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether.)
The aqueous coating composition according to any one of the above [1] to [9], comprising at least one compound selected from the group consisting of compounds represented by the following formula:
[11]
A step of applying the aqueous coating composition according to any one of the above [1] to [10] to a substrate to form an uncured coating film;
and heating the uncured coating film at a temperature of 75° C. or higher and 100° C. or lower to obtain a cured coating film.

The present invention provides a one-component water-based coating composition that has excellent storage stability and can be cured at low temperatures, and a method for producing a coated article that has excellent water resistance.

[Water-based paint composition]
In this embodiment, a carbodiimide compound is used as a curing agent for the aqueous resin, and a basic compound is used in combination with the carbodiimide compound. The basic compound neutralizes the carboxyl groups of the aqueous resin, thereby suppressing the reaction with the carbodiimide compound.

The pH of the aqueous coating composition is adjusted to 8.5 or more and less than 10. By setting the pH to 8.5 or more, the reaction of the carboxyl groups of the aqueous resin in the aqueous coating composition is suppressed, and the aqueous coating composition becomes stable for a long period of time (e.g., one month or more).

By having the pH of the aqueous paint composition be less than 10, the effect on the components contained in the aqueous paint composition is reduced, and the desired coating film can be obtained. For example, by having a pH of less than 10, blackening of aluminum pieces that may be contained as a luster pigment is suppressed.

In addition, the glass transition point (Tg) of the aqueous resin is set to 0°C or higher and 20°C or lower. This makes it possible to improve the physical properties of the resulting coating film while ensuring low-temperature curing properties in a system that uses a carbodiimide compound as a curing agent. For example, the aqueous coating composition according to this embodiment can be cured at low temperatures (e.g., 100°C or lower), and the resulting coating film has excellent water resistance.

That is, the aqueous coating composition according to this embodiment is a one-component type, and contains (A) an aqueous resin having a hydroxyl group and a carboxyl group, (B) a hydrophilically modified carbodiimide compound, and (C) a basic compound. The glass transition point of (A) the aqueous resin is 0°C or higher and 20°C or lower. The pH of the aqueous coating composition is 8.5 or higher and lower than 10.

(B) The hydrophilically modified carbodiimide compound has at least one carbodiimide group and a structure in which a hydroxyl group has been removed from a polyalkylene glycol monoalkyl ether in the molecule.

The aqueous coating composition does not contain a polyisocyanate curing agent, or contains less than 1 part by mass per 100 parts by mass of the solid content of the aqueous coating composition. The polyisocyanate curing agent can react with the hydroxyl groups of the aqueous resin. Since such a curing agent is hardly contained, storage stability is further improved.

After preparation, the aqueous coating composition according to this embodiment can be used even if it is stored at room temperature (between 20°C and 25°C) for one to two months, and can form a coating film with excellent water resistance.

The pH of the aqueous paint composition may be 9.9 or less, or may be 9.8 or less. The pH of typical aqueous paints is about 8.0. The pH is adjusted using a basic compound.

The solids concentration of the aqueous coating composition is not particularly limited. The solids concentration of the aqueous coating composition may be, for example, 15% by mass or more and 60% by mass or less.

The aqueous coating composition according to this embodiment is suitable for forming a colored coating film. The colored coating film may be a so-called intermediate coating film of one or more layers, a so-called base coating film of one or more layers, or a combination of these.

(A) Aqueous resin having hydroxyl group and carboxy group Aqueous resin is a resin that can be dissolved in water or can be dispersed in water. The resin used in the present embodiment has a carboxy group, and therefore has water-solubility or water-dispersibility. (A) Aqueous resin may be water-dispersible.

The volume average particle diameter of the aqueous resin (A) dispersed in the aqueous medium is, for example, 0.01 μm or more and 1 μm or less. Within this range, the resin can be stably dispersed in water, and the appearance of the resulting coating film can be improved. The volume average particle diameter of the aqueous resin (A) is adjusted by the composition of the raw material monomers and/or the polymerization conditions.

The carboxyl group of the water-based resin (A) reacts with the hydrophilically modified carbodiimide compound (B) to form a crosslinked structure. This improves the coating film performance (especially water resistance).

The Tg of the (A) water-based resin is 0°C or higher and 20°C or lower. If the Tg of the (A) water-based resin is too low, components with small molecular weights such as water will easily penetrate the coating film, resulting in a decrease in water resistance. Furthermore, the strength of the resulting coating film will be reduced, and it will be difficult to mold the coating film. If the (A) water-based resin has an excessively high Tg, the volume shrinkage during the cooling process after curing will be large, generating internal stress. The internal stress reduces the adhesion of the coating film. In particular, if the Tg of the (A) water-based resin is higher than the curing temperature, the stress is difficult to relieve, which may cause the coating film to crack. Therefore, in a low-temperature curing aqueous paint composition, it is important to set the Tg of the coating film-forming resin. If the (A) water-based resin has a Tg of 0°C or higher and 20°C or lower, a coating film that can be cured at low temperatures and has excellent water resistance will be obtained.

The Tg of the (A) aqueous resin may be 3°C or higher, or 5°C or higher. The Tg of the (A) aqueous resin may be 18°C or lower, or 15°C or lower.

(A) The Tg of the aqueous resin can be calculated from the type and amount of the raw material monomers. (A) The Tg of the aqueous resin can also be measured by a differential scanning calorimeter (DSC).

(A) The aqueous resin has, for example, a hydroxyl value of 20 mgKOH/g or more and 100 mgKOH/g or less, calculated as the resin solid content. This is to provide adhesion to the substrate. The hydroxyl value may be 30 mgKOH/g or more and 80 mgKOH/g or less.

(A) The aqueous resin has, for example, an acid value of 5 mgKOH/g or more and 80 mgKOH/g or less in terms of resin solid content. This can improve the reaction efficiency with (B) the hydrophilic modified carbodiimide compound, and also makes it easier to increase storage stability. The acid value may be 10 mgKOH/g or more and 60 mgKOH/g or less.

The number average molecular weight of the (A) aqueous resin is, for example, 50,000 or more. The number average molecular weight of the (A) aqueous resin may be 200,000 or more, or 300,000 or more.

The number average molecular weight is measured by gel permeation chromatography (GPC) using polystyrene standard samples.

The content of (A) the aqueous resin is, for example, 30 parts by mass or more and 75 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition. By having the aqueous resin with a relatively large molecular weight account for a large proportion of the solid content, it becomes easier to suppress the migration of the constituent components during the formation of the coating film. As a result, the formation of unevenness is suppressed, and the design can be improved.

The content of (A) the aqueous resin may be 40 parts by mass or more, or 50 parts by mass or more, per 100 parts by mass of the solid content of the aqueous coating composition. The content of (A) the aqueous resin may be 95 parts by mass or less, or 90 parts by mass or less, per 100 parts by mass of the solid content of the aqueous coating composition.

(A) The total content of components having a number average molecular weight of 100,000 or more, such as aqueous resins (hereinafter referred to as high molecular weight components), may be 60 parts by mass or more and 95 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition.

From the viewpoint of design, it is desirable to have a small content of components with low molecular weight. For example, the content of components having a number average molecular weight of 1,000 or less (hereinafter referred to as low molecular weight components) may be 30 parts by mass or less, 20 parts by mass or less, or 10 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition.

The type of (A) aqueous resin is not particularly limited. The (A) aqueous resin may be a single resin or a mixture of different resins. The (A) aqueous resin may be at least one of an acrylic resin and a polyester resin having a hydroxyl group and a carboxyl group, in terms of ease of production or availability. The (A) aqueous resin may contain at least the above-mentioned acrylic resin, in terms of ease of adjusting the coating film properties.

When the aqueous paint composition is for use as an undercoat, the aqueous resin (A) may be, for example, a mixture of the above-mentioned acrylic resin and polyester resin. The ratio of the acrylic resin to the polyester resin may be acrylic resin/polyester resin = 5/1 to 1/1. When the aqueous paint composition is for use as a base coat, the aqueous resin (A) may be, for example, the above-mentioned acrylic resin.

(Acrylic resin having hydroxyl and carboxyl groups)
The acrylic resin having a hydroxyl group and a carboxyl group can be obtained, for example, by acrylic copolymerization of an α,β-ethylenically unsaturated monomer having a hydroxyl group (hereinafter sometimes referred to as a first monomer) and an α,β-ethylenically unsaturated monomer having a carboxyl group (hereinafter sometimes referred to as a second monomer).

Examples of the first monomer include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, allyl alcohol, methacrylic alcohol, and an adduct of hydroxyethyl (meth)acrylate and ε-caprolactone. The first monomer may be at least one selected from the group consisting of 2-hydroxyethyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, and an adduct of 2-hydroxyethyl (meth)acrylate and ε-caprolactone.

"(Meth)acrylic" means both acrylic and methacrylic.

Examples of the second monomer include acrylic acid, methacrylic acid, acrylic acid dimer, crotonic acid, 2-acryloyloxyethyl phthalic acid, 2-acryloyloxyethyl succinic acid, ω-carboxy-polycaprolactone mono(meth)acrylate, maleic acid, fumaric acid, and itaconic acid. The second monomer may be acrylic acid and/or methacrylic acid.

During acrylic copolymerization, other α,β-ethylenically unsaturated monomers (hereinafter sometimes referred to as third monomers) may be used as necessary. Examples of the third monomer include (meth)acrylic acid esters (e.g., methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl methacrylate, phenyl acrylate, isobornyl (meth)acrylate, cyclohexyl methacrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentadienyl (meth)acrylate, dihydrodicyclopentadienyl (meth)acrylate, etc.), and polymerizable amide compounds (e.g., (meth)acrylamide, N-methylol (meth)acrylamide, N-butoxymethyl (meth)acrylamide, etc.).

The method for preparing (A) aqueous resin is not particularly limited. For example, (A) aqueous resin can be prepared by a method in which a polymer is obtained by solution polymerization and then made aqueous, or by a method in which emulsion polymerization is carried out in an aqueous medium to obtain an emulsion.

<Solution polymerization>
The solution polymerization is carried out, for example, by dropping a mixture of the first and second monomers together with a polymerization initiator into a stirred solvent under heating conditions. The solution polymerization is carried out, for example, under conditions of a temperature of 60° C. to 160° C. and a dropping time of 0.5 to 10 hours. The first and second monomers may be polymerized in two stages.

The polymerization initiator is not particularly limited as long as it is one that is used in normal polymerization. Examples include azo compounds and peroxides. The amount of polymerization initiator per 100 parts by mass of the monomer mixture is, for example, 0.1 parts by mass or more and 18 parts by mass or less, and may be 0.3 parts by mass or more and 12 parts by mass or less.

The solvent is not particularly limited as long as it does not adversely affect the reaction. Examples of the solvent include alcohols, ketones, ethers, and hydrocarbon solvents. Furthermore, a chain transfer agent may be added to adjust the molecular weight. Examples of the chain transfer agent include mercaptans such as lauryl mercaptan and α-methylstyrene dimer.

The acrylic resin obtained by solution polymerization is neutralized with a basic compound. This results in a water-dispersible acrylic resin. As the basic compound, those described below can be used. Among them, the basic compound for water dispersion may be ammonia, methylamine, ethylamine, dimethylamine, diethylamine, trimethylamine, triethylamine, dimethylethanolamine, diethanolamine, diethylaminoethanol, or triethanolamine. The basic compound is added in an amount such that the neutralization rate of the carboxyl groups in the acrylic resin obtained by solution polymerization is 60% or more and 100% or less. When the neutralization rate is within this range, water dispersibility is improved.

The neutralization rate is equivalent to the amount of the basic compound used relative to the carboxyl group. The amount of the basic compound used is calculated according to the following formula:

The neutralization of the carboxyl groups is a treatment that is usually carried out to make a carboxyl group-containing resin water-based. However, the water-based resin neutralized in this way is used in an aqueous paint composition having a pH of about 8.0. If the pH is about 8.0, it is difficult to maintain the carboxyl groups in a neutral state (e.g., -COO ^- _NH4 ⁺ ), and improvement in storage stability cannot be expected.

<Emulsion polymerization>
The emulsion polymerization is carried out, for example, by dissolving an emulsifier in an aqueous medium containing water or, if necessary, an organic solvent such as alcohol under heating conditions, and dropping a mixture of the first and second monomers together with a polymerization initiator while heating and stirring the mixture. The monomer mixture may be emulsified in advance using an emulsifier and water.

When emulsion polymerization is performed, a crosslinkable monomer can be used as the third monomer. The crosslinkable monomer has two or more radically polymerizable ethylenically unsaturated groups in the molecule. Examples include divinylbenzene, allyl (meth)acrylate, and ethylene glycol di(meth)acrylate.

Polymerization initiators include azo-based oily compounds such as azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), and 2,2'-azobis(2,4-dimethylvaleronitrile); anionic aqueous compounds such as 4,4'-azobis(4-cyanovaleric acid), 2,2-azobis(N-(2-carboxyethyl)-2-methylpropionamidine, and cationic aqueous compounds such as 2,2'-azobis(2-methylpropionamidine); redox-based oily peroxides such as benzoyl peroxide, parachlorobenzoyl peroxide, lauroyl peroxide, and t-butyl perbenzoate; and aqueous peroxides such as potassium persulfate and ammonium persulfate.

The emulsifier may be a common emulsifier commonly used by those skilled in the art. Examples of emulsifiers include reactive emulsifiers such as Antox MS-60 (manufactured by Nippon Nyukazai Co., Ltd.), Eleminol JS-2 (manufactured by Sanyo Chemical Industries, Ltd.), Adeka Reasoap NE-20 (manufactured by Asahi Denka Co., Ltd.), Aqualon HS-10 (manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), and Latemul PD-104 (manufactured by Kao Corporation). Furthermore, the above-mentioned chain transfer agent may be added to adjust the molecular weight.

The reaction temperature is set according to the polymerization initiator. For example, when an azo-based initiator or peroxide is used, the reaction temperature may be 60°C to 90°C, and when a redox-based initiator is used, the reaction temperature may be 30°C to 70°C. The reaction time is, for example, 1 hour to 8 hours. The amount of the polymerization initiator may be 0.1 parts by mass or more and 5 parts by mass or less per 100 parts by mass of the monomer mixture. The emulsion polymerization may be carried out in multiple stages or in two stages. For example, a part of the monomer mixture may be emulsion-polymerized, and the remainder of the monomer mixture may be added thereto and further emulsion-polymerized.

(Polyester resin having hydroxyl group and carboxy group)
The polyester resin having hydroxyl groups and carboxyl groups can be obtained, for example, by a condensation reaction between a polyhydric alcohol component and a polybasic acid component. The obtained polyester resin is made water-based by neutralizing it with the above-mentioned basic compound.

Examples of polyhydric alcohol components include ethylene glycol, diethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-diethyl-1,3-propanediol, neopentyl glycol, 1,9-nonanediol, 1,4-cyclohexanediol, hydroxypivalic acid neopentyl glycol ester, 2-butyl-2-ethyl-1,3-propanediol, 3-methyl-1,5-pentanediol, and 2,2,4-trimethylpentanediol.

Examples of polybasic acid components include aromatic polycarboxylic acids such as phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, tetrachlorophthalic anhydride, and pyromellitic anhydride; their anhydrides; alicyclic polycarboxylic acids such as hexahydrophthalic anhydride, tetrahydrophthalic anhydride, and 1,4- and 1,3-cyclohexanedicarboxylic acid; their anhydrides; aliphatic polycarboxylic acids such as maleic anhydride, fumaric acid, succinic anhydride, adipic acid, and sebacic acid; their anhydrides.

If necessary, a monobasic acid such as benzoic acid or t-butylbenzoic acid may be used in combination.
As reaction components, monohydric alcohols, monoepoxide compounds such as Cardura E (trade name: manufactured by Shell Chemical Industry Co., Ltd.), and lactones such as β-propiolactone, dimethylpropiolactone, butyrolactone, γ-valerolactone, ε-caprolactone, and γ-caprolactone may be used in combination.

Furthermore, fatty acids such as castor oil, dehydrated castor oil, and mixtures of one or more of these fatty acids may be added to the reaction system.

The polyester resin may be grafted with an acrylic resin and/or a vinyl resin, and may be reacted with a polyisocyanate compound.

(Urethane resin having hydroxyl and carboxyl groups)
As the aqueous resin (A), an aqueous polyurethane resin may be used together with the acrylic resin and/or polyester resin. The aqueous polyurethane resin also has a hydroxyl group and a carboxyl group.

The content of the aqueous polyurethane resin is appropriately set according to the type of coating. When the aqueous coating composition is for undercoating, the content of the aqueous polyurethane resin may be 30 parts by mass or more and 40 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition. When the aqueous coating composition is for base coating, the content of the aqueous polyurethane resin may be 15 parts by mass or more and 30 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition.

(Other resins)
The aqueous coating composition may contain other resins that do not have either or both of a hydroxyl group and a carboxyl group, such as polyether diol, polycarbonate diol, or other resins that have a hydroxyl group, a melamine resin that has a methylol group, or a resin that has a phosphoric acid group.

The content of the other resins is set appropriately depending on the type of coating. When the aqueous coating composition is for a base coating, the content of the other resins may be 15 parts by mass or more and 45 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition.

However, it is desirable to have a low content of resins that only have hydroxyl groups. Such resins do not contribute to curing and are likely to reduce the physical properties of the coating film.

(B) Hydrophilically Modified Carbodiimide Compound (B) The hydrophilically modified carbodiimide compound has, in the molecule, at least one carbodiimide group and a structure in which a hydroxyl group has been removed from a polyalkylene glycol monoalkyl ether.

The hydrophilically modified carbodiimide compound (B) may, for example, have a structure represented by the following formula (α) in the molecule:
-OCONH-X-Q-Y (α)
(In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, each Y is independently a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether, and Q is -NHCOO- or NHCONH-.)
(hereinafter, referred to as structural unit α).

The structural unit α improves water dispersibility and curability. One or more structural units α may be present in one molecule. The number of structural units may be one, two, or three in one molecule.

X is represented, for example, by the following general formula (a).

In the formula, ^R2 is a hydrocarbon group having 6 to 15 carbon atoms. Examples of ^R2 include a phenylene group, a diphenylenemethyl group, a diphenylene(dimethyl)methyl group, a methylphenylene group, a dimethylphenylene group, a tetramethylxylylene group, a hexylene group, a cyclohexylene group, and a dicyclohexylenemethyl group. ^R2 may be a dicyclohexylenemethyl group.

p is 1 to 10. p is the number of carbodiimide groups present in the structural unit. From the viewpoint of curability, p may be 2 or more and 8 or less.

The number of repetitions (e.g., p above) is the average value.

Y is represented, for example, by the following general formula (b) or (c).

In the formula, ^R3 is an alkyl group having 1 to 20 carbon atoms. Examples of ^R3 include a methyl group, an ethyl group, a butyl group, a hexyl group, an octyl group, a decyl group, a dodecyl group, and a stearyl group. From the viewpoint of water dispersibility, ^R3 may be a methyl group.

^R4 is a hydrogen atom or a methyl group. From the viewpoint of water dispersibility, ^R4 is preferably a hydrogen atom.

When ^R4 is hydrogen, general formulas (b) and (c) show the same structure.
q is 4 to 40. From the viewpoints of water dispersibility and reactivity after water volatilization, q may be 4 to 20, or may be 6 to 12.

The ratio Ec/Ea of the equivalent weight Ec of the carbodiimide group of the hydrophilic modified carbodiimide compound (B) to the equivalent weight Ea of the acid group of the aqueous resin (A) is, for example, 0.1 or more and 1.5 or less. When the equivalent ratio is in this range, the crosslinking density is high and the water resistance can be further improved. The ratio Ec/Ea may be 0.5 or more, or 0.8 or more. The ratio Ec/Ea may be 1.4 or less, or 1.2 or less.

The content of the hydrophilically modified carbodiimide compound (B) is, for example, 5 parts by mass or more and 15 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition. When the content of the hydrophilically modified carbodiimide compound (B) is within this range, the water resistance of the coating film can be further improved, and shrinkage due to water is easily suppressed. The above content may be 7 parts by mass or more, or 8 parts by mass or more, per 100 parts by mass of the solid content of the aqueous coating composition. The above content may be 14 parts by mass or less, or 13 parts by mass or less, per 100 parts by mass of the solid content of the aqueous coating composition.

(Hydrophilic modified carbodiimide compound (I))
The hydrophilic modified carbodiimide compound (B) having two structural units α is represented, for example, by the following general formula (I).

In the formula, X and Y are defined as in the above formula (α).
Z has a structure in which a hydroxyl group is removed from a bifunctional polyol having a number average molecular weight of 200 to 5000. Z constitutes a polymer having an ether bond, an ester bond, or a carbonate bond, and is difficult to generalize. For details of Z, see the synthesis method described below.

<Synthesis method>
First, a raw material carbodiimide compound containing at least two isocyanate groups per molecule is reacted with a bifunctional polyol having a hydroxyl group at the molecular end and a number average molecular weight of 200 to 5,000 at a ratio in which the molar amount of the isocyanate group of the raw material carbodiimide compound exceeds the molar amount of the hydroxyl group of the polyol. Next, the resulting reaction product is reacted with a polyalkylene glycol monoalkyl ether. This produces a hydrophilic modified carbodiimide compound (B) represented by formula (I).

From the viewpoint of reactivity, it is preferable that the raw material carbodiimide compound has isocyanate groups at both ends. Methods for producing such raw material carbodiimide compounds are well known to those skilled in the art, and can utilize, for example, a condensation reaction involving the desorption of carbon dioxide from an organic diisocyanate.

Examples of organic diisocyanates include aromatic diisocyanates, aliphatic diisocyanates, alicyclic diisocyanates, and mixtures thereof. Specific examples include 1,5-naphthylene diisocyanate, 4,4-diphenylmethane diisocyanate, 4,4-diphenyldimethylmethane diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, a mixture of 2,4-tolylene diisocyanate and 2,6-tolylene diisocyanate, hexamethylene diisocyanate, cyclohexane-1,4-diisocyanate, xylylene diisocyanate, isophorone diisocyanate, dicyclohexylmethane-4,4-diisocyanate, methylcyclohexane diisocyanate, and tetramethylxylylene diisocyanate. From the standpoint of reactivity, it may be dicyclohexylmethane-4,4-diisocyanate.

A carbodiimidization catalyst is usually used for the condensation reaction. Examples of the carbodiimidization catalyst include phospholene oxides such as 1-phenyl-2-phospholene-1-oxide, 3-methyl-2-phospholene-1-oxide, 1-ethyl-2-phospholene-1-oxide, 3-methyl-1-phenyl-2-phospholene-1-oxide, and 3-phospholene isomers thereof. From the viewpoint of reactivity, 3-methyl-1-phenyl-2-phospholene-1-oxide may be used.

Examples of the bifunctional polyol include polyether diols, polyester diols, and polycarbonate diols. Specific examples include polyalkylene glycols such as polyethylene glycol, polypropylene glycol, polyethylene propylene glycol, polytetramethylene ether glycol, polyhexamethylene ether glycol, and polyoctamethylene ether glycol; polyester diols such as polyethylene adipate, polybutylene adipate, polyhexamethylene adipate, polyneopentyl adipate, poly-3-methylpentyl adipate, polyethylene/butylene adipate, and polyneopentyl/hexyl adipate; polylactone diols such as polycaprolactone diol and poly-3-methylvalerolactone diol; polycarbonate diols such as polyhexamethylene carbonate diol, and mixtures thereof.

The ratio of the molar amount of isocyanate groups in the raw material carbodiimide compound to the molar amount of hydroxyl groups in the bifunctional polyol may be 1.0:1.1 to 1.0:2.0 from the viewpoints of reaction efficiency and economy. The degree of polymerization between the raw material carbodiimide compound and the bifunctional polyol may be 1 to 10 from the viewpoint of reaction efficiency.

The reaction product thus obtained is further reacted with polyalkylene glycol monoalkyl ether (hereinafter referred to as PAGAE). This produces a hydrophilic modified carbodiimide compound (B) having two α structural units.

（式中、Ｒ^３、Ｒ^４、およびｑは、上記と同意義である。） PAGAE is, for example, represented by the following general formula (b') or (c').

(In the formula, R ³ , R ⁴ , and q are defined as above.)

The number average molecular weight of the PAGAE may be 200 or more and 5,000 or less. The number of carbon atoms in the alkyl group of the PAGAE may be 1 to 20. Examples of the PAGAE include polyethylene glycol, polypropylene glycol, or a mixture thereof, each of which is end-blocked with an alkyl group having 1 to 20 carbon atoms. Specific examples of the PAGAE include polyethylene glycol monomethyl ether, polyethylene glycol mono-2-ethylhexyl ether, polyethylene glycol monolauryl ether, polypropylene glycol monomethyl ether, polypropylene glycol mono-2-ethylhexyl ether, and polypropylene glycol monolauryl ether, each of which has a number average molecular weight of 200 to 5,000.

The reaction of the above reaction product with PAGAE is carried out in such a ratio that the molar amount of isocyanate groups in the above reaction product is equal to or exceeds the molar amount of hydroxyl groups in PAGAE. The molar amount of isocyanate groups is determined by direct measurement. The molar amount of isocyanate groups can also be calculated from the amount of charge.

A catalyst may be used during the reaction. The reaction temperature is not particularly limited, but may be 60°C or higher and 120°C or lower from the viewpoint of reaction system control and reaction efficiency. The solvent may be an organic solvent that does not contain active hydrogen.

The hydrophilic modified carbodiimide compound (B) produced through such a two-step reaction may be a mixture of the compound represented by general formula (I) and other reaction products. However, these mixtures may also be considered to be hydrophilic modified carbodiimide compounds (B) having the structure of general formula (I).

(Hydrophilic modified carbodiimide compound (II))
The hydrophilic modified carbodiimide compound (B) having three structural units α is represented, for example, by the following general formula (II).

In the formula, X and Y are defined as in the above formula (α).
^R0 is hydrogen, a methyl group, or an ethyl group. Each ^R1 is independently an alkylene group having 4 or less carbon atoms. Specific examples include a methylene group, an ethylene group, a propylene group, and a butylene group. n is 0 or 1. Each m is independently an integer from 0 to 60.

R ⁰ , R ¹ , n and m are determined depending on the trifunctional polyol used in producing the hydrophilically modified carbodiimide compound (B).

When m is 11 or more, the ratio of hydrophilic parts to hydrophobic parts is preferably 2.0 to 6.3. The above ratio is determined by dividing the molecular weight of the oxymethylene group or oxyethylene group part present in the carbodiimide compound by the molecular weight of the carbodiimide compound.

<Synthesis method>
First, the raw material carbodiimide compound is reacted with the PAGAE in such a ratio that the equivalent of the isocyanate group of the raw material carbodiimide compound exceeds the equivalent of the hydroxyl group of the PAGAE. Then, the resulting reaction product is reacted with a trifunctional polyol. This results in a hydrophilic modified carbodiimide compound (B) represented by general formula (II). The equivalent ratio of the isocyanate group to the hydroxyl group may be 2/1.

The amount of trifunctional polyol may be an amount that results in a hydroxyl equivalent equal to or greater than the isocyanate equivalent of the reactant, and the isocyanate equivalent and the hydroxyl equivalent may be equal. The isocyanate equivalent of the reaction product is determined by direct measurement. The molar amount of isocyanate groups is also calculated from the amount charged.

The reaction can be carried out under conditions that are generally well known to those skilled in the art, and a tin-based catalyst can be used as necessary. The reaction is carried out in the same manner as in the case of the hydrophilic modified carbodiimide compound (B) represented by the general formula (I).

The trifunctional polyol may be trimethylolpropane, glycerin, or an alkylene oxide adduct thereof, because they are easily available. Examples of alkylene oxides include ethylene oxide and propylene oxide. Alkylene oxide adducts of glycerin are commercially available as the GP series from Sanyo Chemical Industries. Considering the curing reactivity of the resulting hydrophilic modified carbodiimide compound, the trifunctional polyol may have a structure in which an alkylene oxide is added to each hydroxyl group. Among the GP series, for example, GP-250 and GP-3000 have such a structure.

The hydrophilic modified carbodiimide compound (B) produced through such a two-step reaction may be a mixture of the compound represented by general formula (II) and other reaction products. However, these mixtures may also be considered to be hydrophilic modified carbodiimide compounds (B) having the structure of general formula (II).

（式中、ＸおよびＹは、上記式（α）と同意義である。） (Hydrophilic modified carbodiimide compound (III))
The hydrophilically modified carbodiimide compound (B) having one structural unit α is represented, for example, by the following general formula (III).

(In the formula, X and Y are defined as in the above formula (α).)

Each Y may independently have a structure selected from the following (i) or (ii). This can improve water dispersibility and stability, and also makes it easier to increase the crosslink density.

(i) A structure in which a hydroxyl group has been removed from a polyethylene glycol monoalkyl ether in which an alkyl group having 1 to 3 carbon atoms is ether-bonded to the end of a polyethylene oxide unit having a repeat number of 6 to 20. (ii) A structure in which a hydroxyl group has been removed from a polypropylene glycol monoalkyl ether in which an alkyl group having 1 to 8 carbon atoms is ether-bonded to the end of a polypropylene oxide unit having a repeat number of 4 to 60.

In particular, Y may have the structure (ii), and the number of repeating polypropylene oxide units may be 15 to 60.

One Y may have the structure (i) and the other Y may have the structure (ii). In this case, the ratio of the number of Y's having the structure (i) to the number of Y's having the structure (ii) may be (i):(ii) = 1:0.7 to 1:8.

From the viewpoint of suppressing the deactivation of the carbodiimide by water and improving the water resistance of the coating film, it is desirable that the area around the carbodiimide group is hydrophobic to a certain extent and has low contact with water molecules. On the other hand, the carbodiimide compound itself is required to have hydrophilicity. When (i):(ii) is in the above range, the hydrophobicity and hydrophilicity are easily balanced. (i):(ii) may be 1:0.7 to 1:1.5.

<Synthesis method>
The hydrophilically modified carbodiimide compound (B) represented by the general formula (III) can be obtained by reacting the above starting carbodiimide compound with the same or a different PAGAE.

The PAGAE may be at least one selected from the group consisting of the following (i') and (ii'):
(i') A polyethylene glycol monoalkyl ether having an alkyl group having 1 to 3 carbon atoms ether-bonded to the end of a polyethylene oxide unit having a repeat number of 6 to 20. (ii') A polypropylene glycol monoalkyl ether having an alkyl group having 1 to 8 carbon atoms ether-bonded to the end of a polypropylene oxide unit having a repeat number of 4 to 60.

Specific examples of (i') include polyethylene glycol monomethyl ether, polyethylene glycol monoethyl ether, and polyethylene glycol monopropyl ether. In particular, it may be polyethylene glycol monomethyl ether.
Specific examples of (ii') include polypropylene glycol monomethyl ether, polypropylene glycol monoethyl ether, polypropylene glycol monobutyl ether, and polypropylene glycol 2-ethylhexyl ether. In particular, polypropylene glycol monobutyl ether may be used.

In terms of crosslink density, (B) the hydrophilically modified carbodiimide compound may be represented by general formula (III).

(Other hardeners)
The aqueous coating composition may contain other curing agents in addition to the hydrophilically modified carbodiimide compound (B). Examples of other curing agents include polyisocyanate curing agents (including blocked polyisocyanate curing agents), amino resins, epoxy compounds, aziridine compounds, and oxazoline compounds.

However, the content of the polyisocyanate curing agent is less than 1 part by mass per 100 parts by mass of the solid content of the aqueous coating composition, and may be 0 parts by mass. This is because the polyisocyanate curing agent reduces storage stability. As mentioned above, from the viewpoint of design, it is desirable that the content of the curing agent corresponding to the low molecular weight component is also small.

(C) Basic Compounds A basic compound is a compound that acts as a base according to the Bronsted-Lowry definition. A basic compound accepts a proton (H ⁺ ).

In the aqueous coating composition, the basic compound neutralizes the carboxyl groups of the aqueous resin (A). This suppresses the reaction between the aqueous resin (A) and the carbodiimide compound (B), improving storage stability and pot life.

The content of the basic compound is not particularly limited as long as the pH of the aqueous coating composition is in the range of 8.5 or more and less than 10. The content of the basic compound may be, for example, 1.0 part by mass or more, 1.2 parts by mass or more, or 1.4 parts by mass or more per 100 parts by mass of the solid content of the aqueous coating composition. The content of (C) the basic compound may be, for example, 10.0 parts by mass or less, 7.0 parts by mass or less, or 5.0 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition.

After the aqueous coating composition is applied to the substrate, the basic compound is required to volatilize. The volatilization of the basic compound allows the carboxyl group of the aqueous resin to react with the carbodiimide compound. The boiling point of the basic compound may be such that it can be volatilized by the heat treatment performed after coating. The boiling point of the basic compound may be, for example, 180°C or lower, 170°C or lower, or 160°C or lower. The boiling point of the basic compound may be such that it does not volatilize during storage of the aqueous coating composition. The boiling point of the basic compound may be, for example, 60°C or higher, 70°C or higher, or 80°C or higher.

The pKa (acid dissociation constant) of the basic compound is, for example, 5.0 or more. This makes it easier to suppress the reaction between the carbodiimide compound and the aqueous resin. The pKa of the basic compound may be 7.0 or more, and may be greater than 8.5. The pKa of the basic compound is, for example, 30.0 or less. This makes it easier to improve the water resistance of the resulting coating film. The pKa of the basic compound may be 20.0 or less, and may be 15.0 or less. In one embodiment, the pKa of the basic compound is 5.0 or more and 30.0 or less. The pKa is a value when water is used as a solvent at 25°C.

The basic compound is not particularly limited, and includes, for example, at least one selected from the group consisting of ammonia (pKa 9.25°C) and amine compounds. Examples of amine compounds include dimethylamine (pKa 10.64), trimethylamine (pKa 9.76), ethylamine (pKa 10.63), diethylamine (pKa 10.98), triethylamine (pKa 10.7), n-propylamine (pKa 10.53), isopropylamine (pKa 10.63), triallylamine (pKa 8.3, boiling point 156°C), triethylenediamine (pKa 8.7, boiling point 174°C), N,N-dimethylethanolamine (boiling point 135°C), N,N-diethylethanolamine, aminoethanolamine, N -Methyl-N,N-diethanolamine, iminobispropylamine, 3-ethoxypropylamine, 3-diethylaminopropylamine, methylaminopropylamine, methyliminobispropylamine, 3-methoxypropylamine, monoethanolamine (pKa 9.5), diethanolamine, triethanolamine, morpholine (pKa 8.4, boiling point 129°C), allylmorpholine (pKa 7.1, boiling point 158°C), N-methylmorpholine (pKa 7.4, boiling point 116°C), and N-ethylmorpholine (pKa 7.7, boiling point 139°C). The basic compound is preferably N,N-dimethylethanolamine.

(others)
The aqueous coating composition may contain, as necessary, for example, a diluent, a pigment, a curing catalyst, a surface conditioner, a defoamer, a pigment dispersant, a plasticizer, a film-forming assistant, an ultraviolet absorber, an antioxidant, and a pH adjuster. The aqueous coating composition of the present invention can be prepared by mixing the components constituting the aqueous coating composition by any commonly used means.

[Method of manufacturing coated objects]
The aqueous coating composition according to the present embodiment can be cured at low temperature. The coated article is produced, for example, by a method including a step of applying the aqueous coating composition according to the present embodiment to a substrate to form an uncured coating film, and a step of heating the uncured coating film at a temperature of 75° C. to 100° C. to obtain a cured coating film. The obtained cured coating film has excellent water resistance.

When a colored coating film is formed using the aqueous coating composition according to this embodiment, the coated article is produced by a method including, for example, a step of applying the aqueous coating composition according to this embodiment to an article to be coated to form an uncured colored coating film, a step of heating the uncured colored coating film at a temperature of 75°C or higher and 100°C or lower to form a cured colored coating film, a step of applying a clear coating composition onto the colored coating film to form an uncured clear coating film, and a step of heating the uncured clear coating film to obtain a cured clear coating film.

When the clear coating film is formed, the colored coating film may be cured or uncured. In particular, from the viewpoints of productivity, adhesion, and water resistance, it is preferable to laminate each coating film without curing it (so-called wet-on-wet coating), and then simultaneously cure these multiple uncured coating films. That is, the coated article may be manufactured by a method including a step of applying the aqueous coating composition according to this embodiment onto the coated article to form an uncured colored coating film, a step of applying the clear coating composition onto the uncured colored coating film to form an uncured clear coating film, and a step of curing the uncured colored coating film and the uncured clear coating film.

The following describes the case where the aqueous coating composition according to this embodiment is used to form a colored coating film and is applied wet-on-wet. However, the use of the aqueous coating composition according to this embodiment is not limited to this.

(1) Step of forming an uncured colored coating film The aqueous coating composition according to this embodiment is applied to a substrate. This forms an uncured colored coating film. This step may be repeated multiple times before applying the clear coating composition. This forms multiple uncured colored coating films.

The colored coating film may be one layer, or may be a laminated coating film of two or more layers. The colored coating film may be one or more layers of a so-called intermediate coating film, one or more layers of a so-called base coating film, or a combination of these. The intermediate coating film is typically formed adjacent to the substrate. The base coating film is typically formed on the intermediate coating film.

(Substrate)
Examples of the substrate include steel plates of metals such as iron, steel, stainless steel, aluminum, copper, zinc, and tin, and alloys thereof; polyethylene resin, EVA resin, polyolefin resin (polyethylene resin, polypropylene resin, etc.), vinyl chloride resin, styrene resin, polyester resin (including PET resin, PBT resin, etc.), polycarbonate resin, acrylic resin, acrylonitrile butadiene styrene (ABS) resin, acrylonitrile styrene (AS) resin, polyamide resin, acetal resin, phenol resin, fluororesin, melamine resin, urethane resin, epoxy resin, polyphenylene oxide (PPO), and other resins; and organic-inorganic hybrid materials. The substrate may be flat or have a three-dimensional shape.

Metallic substrates may be preliminarily subjected to chemical conversion treatment and electrocoating. Examples of chemical conversion treatments include zinc phosphate conversion treatment, zirconium conversion treatment, and chromium oxide conversion treatment. Electrocoating may be cationic electrocoating or anionic electrocoating.

The resin substrate may be degreased beforehand. After degreasing, the resin substrate may be further coated with a primer paint. There are no particular limitations on the primer paint, and it may be selected appropriately depending on the type of paint to be applied on top of it.

(Colored coating film)
The aqueous coating composition used to form a colored coating film contains a pigment, such as a color pigment, a luster pigment, and an extender pigment.

Examples of color pigments include organic color pigments such as azo chelate pigments, insoluble azo pigments, condensed azo pigments, diketopyrrolopyrrole pigments, benzimidazolone pigments, phthalocyanine pigments, indigo pigments, perinone pigments, perylene pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, and metal complex pigments; and inorganic color pigments such as yellow lead, yellow iron oxide, red iron oxide, carbon black, and titanium dioxide. These may be used alone or in combination of two or more.

Examples of photoluminescent pigments include metal flakes (aluminum, chromium, gold, silver, copper, brass, titanium, nickel, nickel chrome, stainless steel, etc.), metal oxide flakes, pearl pigments, glass flakes coated with metal or metal oxide, silica flakes coated with metal oxide, graphite, holographic pigments, and cholesteric liquid crystal polymers. These may be used alone or in combination of two or more.

Examples of extender pigments include calcium carbonate, barium sulfate, clay and talc. These may be used alone or in combination of two or more.

The concentration of all pigments, i.e., the mass ratio (PWC) of all pigments to 100% by mass of resin solids in the aqueous paint composition, is preferably 0.1% by mass or more and 50% by mass or less. This improves the smoothness of the resulting coating film. The PWC of each pigment is not particularly limited. The PWC of the luster pigment may be, for example, 1% by mass or more and 40% by mass or less. The PWC of the luster pigment is preferably 5% by mass or more. The PWC of the luster pigment is preferably 30% by mass or less.

The coating method is not particularly limited. Examples of coating methods include air spray coating, airless spray coating, rotary atomization coating, and curtain coat coating. These methods may be combined with electrostatic coating. Among them, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency. For rotary atomization electrostatic coating, for example, rotary atomization electrostatic coating machines commonly known as "micro-microbell (μμbell)", "microbell (μbell)", "metallicbell (metabell)", etc. are used.

After applying the aqueous coating composition, preliminary drying (also called preheating) may be performed. At this time, the basic compound may volatilize. In addition, bumping of the diluting components is suppressed, which helps to prevent popping. Furthermore, mixing of the uncured colored coating film with the coating composition applied thereon is suppressed, which makes it difficult for a mixed phase to form. This makes it easier to improve the smoothness of the resulting coated article.

There are no particular limitations on the conditions for pre-drying. Examples of pre-drying include leaving the material at a temperature of 20°C to 25°C for 15 to 30 minutes, or heating the material at a temperature of 50°C to 100°C for 30 seconds to 10 minutes.

The thickness of the intermediate coating film is not particularly limited. In terms of the smoothness and chipping resistance of the coated article, the thickness of each layer of the intermediate coating film after curing is preferably 5 μm or more and 40 μm or less. The thickness of each layer of the intermediate coating film after curing is more preferably 15 μm or more. The thickness of each layer of the intermediate coating film after curing is more preferably 30 μm or less.

The thickness of the base coating film is not particularly limited and is set appropriately depending on the purpose. The thickness of each layer of the base coating film after curing is, for example, 10 μm or more, may be 15 μm or more, or may be 20 μm or more. The thickness of each layer of the base coating film after curing is, for example, 50 μm or less, may be 45 μm or less, or may be 40 μm or less.

(2) Step of forming an uncured clear coating film A clear coating composition is applied onto the uncured colored coating film, thereby forming an uncured clear coating film.

The clear coating composition is not particularly limited. The clear coating composition contains, for example, a hydroxyl-containing resin and a polyisocyanate curing agent. The hydroxyl value of the hydroxyl-containing resin may be 20 mgKOH/g or more and 200 mgKOH/g or less. The number average molecular weight of the hydroxyl-containing resin may be 1,000 or more and 20,000 or less. The polyisocyanate curing agent is not particularly limited, and examples thereof include aliphatic isocyanates, aliphatic cyclic isocyanates, aromatic isocyanates, alicyclic isocyanates, and their biuret and nurate polymers and mixtures.

Commercially available clear coating compositions can also be used. Examples of commercially available clear coating compositions include Polyurexel O-1100 Clear and O-1200 Clear (manufactured by Nippon Paint Automotive Coatings Co., Ltd., isocyanate-curing clear coating compositions).

The coating method is not particularly limited. Examples of coating methods include the same methods as those for coating the aqueous coating composition described above. Among them, rotary atomization electrostatic coating is preferred from the viewpoint of coating efficiency. After coating the clear coating composition, preliminary drying may be performed. The conditions for preliminary drying are not particularly limited and may be the same as those for preliminary drying of the colored coating film.

(4) Curing Step Each uncured coating film is heated at a temperature of 75° C. or more and 100° C. or less.
The basic compound (C) volatilizes in at least one step selected from the group consisting of pre-drying for the colored coating film, pre-drying for the clear coating film, and heating in the curing step. That is, the cation derived from the basic compound is removed from the neutralized carboxyl group of the aqueous resin, and the carboxyl group is regenerated. This causes the aqueous resin to react with the hydrophilic modified carbodiimide compound, and the coating composition is cured.

The curing temperature may be 90°C or less. However, the aqueous coating composition according to this embodiment may be cured at a higher temperature. For example, the curing temperature may be greater than 100°C and less than or equal to 120°C.

The heating time may be set appropriately depending on the heating temperature. For example, the heating time may be from 5 to 60 minutes, or from 10 to 45 minutes. The heating time means the time during which the inside of the heating device is kept at the target temperature, and does not take into account the time it takes to reach the target temperature. Examples of heating devices include drying furnaces that use heat sources such as hot air, electricity, gas, and infrared rays.

The present invention will be described in more detail with reference to the following examples, but is not limited thereto. In the examples, "parts" and "%" are by weight unless otherwise specified.

(Preparation of Acrylic Emulsion 1 Having Hydroxyl Groups and Carboxy Groups)
A reaction vessel equipped with a stirrer, a nitrogen inlet tube, a temperature control device, a condenser, and a dropping funnel was charged with 607 parts of deionized water and 5 parts of LATEMULL PD-104 (Kao Corporation, 20% aqueous solution) as an emulsifier, and the temperature was raised to 80°C with stirring under a nitrogen atmosphere.
Separately, 191 parts of styrene, 61 parts of methyl methacrylate, 179 parts of 2-ethylhexyl acrylate, 46 parts of hydroxyethyl acrylate, 8 parts of methacrylic acid, and 15 parts of allyl methacrylate were added to an emulsifier aqueous solution prepared by mixing 45 parts of LATEMURU PD-104 in 271 parts of deionized water, and emulsified to obtain a pre-emulsion.

This pre-emulsion was added dropwise to a reaction vessel over a period of 2 hours together with an aqueous initiator solution in which 1.5 parts of ammonium persulfate was dissolved in 204 parts of deionized water. After the addition was completed, the reaction was continued for a further 2 hours at 80°C. It was then cooled and 0.8 parts of N,N-dimethylamino ether (equivalent to 10% of the acid value of the resin (neutralization rate 10%)) was added to obtain acrylic emulsion 1 with a resin solid content of 30% by mass.

The hydroxyl value of Acrylic Emulsion 1 calculated from the monomer composition, calculated as the resin solid content, was 40 mg KOH/g and the acid value was 10 mg KOH/g.

(Production of hydrophilically modified carbodiimide compound)
700 parts of 4,4-dicyclohexylmethane diisocyanate and 7 parts of 3-methyl-1-phenyl-2-phospholene-1-oxide were reacted at 170°C for 7 hours to obtain a carbodiimide compound having a structure represented by the above general formula (a), three carbodiimide groups per molecule, and isocyanate groups at both ends.

Next, 95 parts of PTMG-1000 (polytetramethylene glycol with a number average molecular weight of 1,000, manufactured by Mitsubishi Chemical Corporation, with 13.6 repeating units of tetramethylene oxide calculated from the number average molecular weight) and 0.2 parts of dibutyltin dilaurate were added to 180 parts of the produced carbodiimide compound, and the mixture was heated to 85°C and maintained at this temperature for 2 hours.

Next, 86.4 parts of Methyl Polyglycol 130 (polyethylene glycol monomethyl ether manufactured by Nippon Nyukazai Co., Ltd., 9 repeat units of ethylene oxide calculated from a hydroxyl value of 130 mg KOH/g) was added and the mixture was kept at 85°C for 3 hours. The reaction was terminated when it was confirmed by IR measurement that the NCO peak had disappeared, and the mixture was cooled to 60°C. Deionized water was then added to obtain an aqueous dispersion of a hydrophilic modified carbodiimide compound with a resin solid content of 40% by mass. The obtained hydrophilic modified carbodiimide compound was a compound represented by the above general formula (I).

(Preparation of color pigment paste)
9.2 parts of a commercially available dispersant "Disperbyk 190" (manufactured by BYK-Chemie), 17.8 parts of ion-exchanged water, and 73.0 parts of rutile-type titanium dioxide were premixed. Then, using a paint conditioner, the premix was mixed and dispersed at room temperature together with a bead medium until the particle size became 5 μm or less, to obtain a color pigment paste.

[Example 1]
(1) Preparation of aqueous base coating composition In a container having a stirrer, 100 parts of acrylic emulsion 1 (resin solid content 30%), 10 parts of hydrophilic modified carbodiimide compound (resin solid content 40%), 10 parts of Alpaste MH8801 (Asahi Kasei aluminum pigment, solid content 65%) as a glitter pigment (PWC 12%), 0.3 parts of lauryl acid phosphate were added, and further, 30 parts of 2-ethylhexanol, 2 parts of Adekanol UH-814N (ADEKA thickener, solid content 30%), 1 part of N,N-dimethylethanolamine (Kishida Chemical Co., Ltd.), and 50 parts of ion-exchanged water were uniformly dispersed to obtain an aqueous base coating composition. The pH of the aqueous base coating composition was 9.5.

(2) Formation of multilayer coating film A zinc phosphate-treated dull steel plate was electrocoated with Powernics 150 (product name, cationic electrodeposition paint manufactured by Nippon Paint Automotive Coatings Co., Ltd.) so that the dry coating film had a thickness of 20 μm. The plate was then heat-cured at 160° C. for 30 minutes and cooled to prepare a steel plate substrate.
The above-mentioned water-based base coating material was applied to the obtained substrate by a rotary atomizing electrostatic coating device so that the dry film thickness was 15 μm, and then the substrate was preheated at 80° C. for 3 minutes.

The plate was then coated with a clear coating (PolyureExcel O-1200, manufactured by Nippon Paint Automotive Coatings Co., Ltd., a two-liquid acrylic urethane organic solvent-based clear coating containing a polyisocyanate compound) using a rotary atomizing electrostatic coating device to a dry thickness of 35 μm. After that, the plate was heated and cured at 80°C for 20 minutes to obtain a coated object with a cured multi-layer coating.

[Examples 2 to 4, Comparative Examples 1 to 5]
An aqueous base coating composition was prepared and a coated article was obtained in the same manner as in Example 1, except that the composition of the aqueous base coating composition was changed to the components and amounts thereof shown in the table below.

[evaluation]
The following evaluation tests were carried out on the aqueous base coating composition and the coated object. The evaluation results are shown in Table 1.

(1) Storage stability After preparation of the coating composition, it was left to stand for 10 days at 40° C. If a non-fluid lump was formed in the coating composition or if the viscosity of the entire coating composition was 0% or less or 150% or more of the initial viscosity, it was rated as "poor", and if not, it was rated as "good".

(2) Water resistance The coated object was immersed in 40°C warm water for 240 hours. The coated object was then removed and dried at 20°C for 24 hours. The multi-layer coating film of the coated object was then cut into a grid shape with a cutter until it reached the base material, and divided into 100 grids measuring 2 mm x 2 mm. Next, adhesive cellophane tape ^(trademark) was applied to the surface of the grid, and the tape was rapidly peeled off at 20°C. The number of grids from which the multi-layer coating film had peeled off was counted. The smaller this number, the better the water resistance.
Good: No peeled squares Bad: One or more peeled squares

The aqueous coating composition of the embodiment has excellent storage stability, and even when cured at a low temperature of 80°C, it can form a coating film with excellent water resistance.

On the other hand, the aqueous coating composition of Comparative Example 1 does not contain a hydrophilic modified carbodiimide compound, and therefore has excellent storage stability. However, since the Tg of the aqueous resin is less than 0°C, the water resistance of the multi-layer coating film is low. The aqueous coating composition of Comparative Example 2 has a pH of less than 8.5, and therefore has poor storage stability. Furthermore, since the Tg of the aqueous resin is less than 0°C, the water resistance of the multi-layer coating film is also low. The aqueous coating composition of Comparative Example 3 has excellent storage stability, but since the pH is high, the water resistance of the multi-layer coating film is low. The aqueous coating compositions of Comparative Examples 4 and 5 have a pH of 8.5 or more and less than 10, and therefore have excellent storage stability. On the other hand, since the Tg of the aqueous resin is not in the range of 0 to 20°C, the water resistance of the multi-layer coating film is low.

The aqueous coating composition of the present invention has excellent storage stability and can be cured at low temperatures, making it suitable for a variety of applications.

Claims (11)

A one-component aqueous coating composition,
(A) an aqueous resin having a hydroxyl group and a carboxyl group;
(B) a hydrophilically modified carbodiimide compound;
(C) a basic compound,
The glass transition point of the (A) aqueous resin is 0° C. or higher and 20° C. or lower,
The hydrophilically modified carbodiimide compound (B) has at least one carbodiimide group and a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether in the molecule,
The pH of the aqueous coating composition is 8.5 or more and less than 10,
The aqueous coating composition does not contain a polyisocyanate curing agent or contains less than 1 part by mass of a polyisocyanate curing agent per 100 parts by mass of the solid content of the aqueous coating composition. The aqueous coating composition according to claim 1, wherein the content of the basic compound (C) is 1 part by mass or more and 10 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition. The aqueous coating composition according to claim 1 or 2, wherein the boiling point of the basic compound (C) is 180°C or lower. The aqueous coating composition according to any one of claims 1 to 3, wherein the (C) basic compound includes at least one selected from the group consisting of ammonia and amine compounds. The aqueous coating composition according to any one of claims 1 to 4, wherein the aqueous resin (A) has a hydroxyl value of 20 mgKOH/g or more and 100 mgKOH/g or less, and an acid value of 5 mgKOH/g or more and 80 mgKOH/g or less, calculated as the resin solid content. The aqueous coating composition according to any one of claims 1 to 5, wherein the ratio of the equivalent weight Ec of the carbodiimide group of the hydrophilically modified carbodiimide compound (B) to the equivalent weight Ea of the acid group of the aqueous resin (A): Ec/Ea is 0.1 or more and 1.5 or less. The aqueous coating composition according to any one of claims 1 to 6, wherein the content of the aqueous resin (A) is 30 parts by mass or more and 75 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition. The aqueous coating composition according to any one of claims 1 to 7, wherein the content of the hydrophilically modified carbodiimide compound (B) is 5 parts by mass or more and 15 parts by mass or less per 100 parts by mass of the solid content of the aqueous coating composition. The hydrophilically modified carbodiimide compound (B) is represented by the following general formula (α):
-OCONH-X-Q-Y (α)
(In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, each Y is independently a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether, and Q is -NHCOO- or NHCONH-.)
The aqueous coating composition according to any one of claims 1 to 8, which has one or more structural units represented by the following formula: The hydrophilically modified carbodiimide compound (B) is
The following general formula (I):

(In the formula, X's are each independently a bifunctional organic group containing at least one carbodiimide group, Y's are each independently a polyalkylene glycol monoalkyl ether having a structure in which a hydroxyl group has been removed, and Z's are each independently a bifunctional polyol having a number average molecular weight of 200 or more and 5,000 or less having a hydroxyl group removed.)
The following general formula (II):

(wherein, X's are each independently a bifunctional organic group containing at least one carbodiimide group, Y's are each independently a polyalkylene glycol monoalkyl ether having a hydroxyl group removed therefrom, ^R0 is hydrogen, a methyl group, or an ethyl group, ^R1 's are each independently an alkylene group having 4 or less carbon atoms, n is 0 or 1, and m is 0 to 60), and
The following general formula (III):

(In the formula, each X is independently a bifunctional organic group containing at least one carbodiimide group, and each Y is independently a structure obtained by removing a hydroxyl group from a polyalkylene glycol monoalkyl ether.)
The aqueous coating composition according to any one of claims 1 to 9, comprising at least one compound selected from the group consisting of compounds represented by the following formula: A step of applying the aqueous coating composition according to any one of claims 1 to 10 to a substrate to form an uncured coating film;
and heating the uncured coating film at a temperature of 75° C. or higher and 100° C. or lower to obtain a cured coating film.